I have a 1972 RT2.  On the top of the piston, the number "77" is stamped by the arrow and opposite are stamped the numbers "968". What size piston is this?  I need to bore what is the next size?  Are pistons still available?

Thanks,

Almost certainly standard bore piston - others will explain what the numbers mean.

Oversizes go up in 0.25mm increments, and Yamaha oversize pistons will have .25, .5 etc marked on the crown.

Check fleabay for original parts -DOUBLE check what they are advertising is correct, as I've seen some listings say "fits 250 & 360"  .

CMSNL and Partzilla will give the part numbers for the oversize pistons and rings.

Your engineer should tell you which size up to go - eg deep scoring might necessitate two sizes up.

It'll still be std 80mm which is pretty amazing after 50 years. First over is 0.25mm or 10 thou larger. Just ask your machine shop if it'll clean up at 'that' or if you need to get a 0.50mm or 20 thou over kit.

Thanks for the help.